In the field of industrial process measurements technology, especially also in connection with the automation of chemical or manufacturing processes, for registering process-describing, measured variables, and for producing measurement signals representing these, measuring systems installed near the process are used. Such measuring systems are mounted either directly on, or in, a process line, for instance a pipeline, through which medium is flowing. The variables to be measured can include, for example, mass flow, volume flow, flow velocity, density, viscosity, temperature, or the like, of a liquid, powdered, vaporous, or gaseous, process medium conveyed or held in such a process line.
Such measuring systems include, among others, those in which in-line measuring devices with magneto-inductive transducers are used, or transducers which evaluate the travel time of ultrasonic waves emitted in the flow direction, especially measuring transducers functioning according to the Doppler principle, or vibration-type transducers, especially Coriolis mass flow transducers, density transducers, or the like. Essentials of construction and functioning of magneto-inductive sensors is sufficiently described e.g. in EP-A 1 039 269, U.S. Pat. Nos. 6,031,740, 5,540,103, 5,351,554, 4,563,904; for ultrasonic sensors see e.g. U.S. Pat. Nos. 6,397,683, 6,330,831, 6,293,156, 6,189,389, 5,531,124, 5,463,905, 5,131,279, 4,787,252. Moreover, this background on these topics is adequately known to those skilled in the art, so that a detailed explanation can be omitted here. Further examples of such measuring systems known per se to those skilled in the art, especially systems made up of compact, in-line measuring devices, are explained in detail in EP-A 984 248, GB-A 21 42 725, U.S. Pat. Nos. 4,308,754, 4,420,983, 4,468,971, 4,524,610, 4,716,770, 4,768,384, 5,052,229, 5,052,230, 5,131,279, 5,231,884, 5,359,881, 5,458,005, 5,469,748, 5,687,100, 5,796,011, 5,808,209, 6,003,384, 6,053,054, 6,006,609, 6,352,000, 6,397,683, 6,513,393, 6,644,132, 6,651,513, 6,880,410, 6,910,387, US-A 2007/0163361, US-A 2005/0092101, WO-A 88/02,476, WO-A 88/02,853, WO-A 95/16,897, WO-A 00/36,379, WO-A 00/14,485, WO-A 01/02816 or WO-A 02/086 426, among others.
For registering the respective measured variables, measuring systems of the type in discussion here include a corresponding transducer, which is inserted into the course of a process line conveying a medium, in order to produce at least one measurement signal, especially an electric signal, representing the primarily measured variable as accurately as possible. To this end, the measuring transducer normally includes a measuring tube, which is inserted into the course of the pipeline and serves for conveying flowing medium, and a corresponding physical-to-electrical sensor arrangement. The sensor arrangement, in turn, includes at least one sensor element reacting primarily to the variable to be measured, or to changes of the same, in order to produce, during operation, at least one measurement signal appropriately influenced by the measured variable.
For further processing or evaluation of the at least one measurement signal, the transducer is additionally connected with a measuring electronics appropriately suited for this purpose. The measuring electronics, communicating with the measuring transducer in suitable manner, produces, during operation of the measuring system, at least at times, using the at least one measurement signal, at least one measured value instantaneously representing the measured variable, thus, for example, a measured value of mass flow, volume flow, density, viscosity, pressure, temperature, or the like.
To accommodate the measuring electronics, such measuring systems additionally include an appropriate electronics housing, which, as proposed e.g. in U.S. Pat. No. 6,397,683 or WO-A 00/36 379, can be arranged remotely from the measuring transducer and connected with it only by means of a flexible cable. Alternatively, however, as also shown e.g. in EP-A 903 651 or EP-A 1 008 836, for forming a compact in-line measuring device (for example a Coriolis mass flow/density measuring device, an ultrasonic flow-measuring device, a vortex flow-measuring device, a thermal flow-measuring device, a magneto-inductive flow-measuring device, or the like), the electronics housing can be arranged directly on the measuring transducer, or on a transducer housing separately housing the measuring transducer. In the latter case, the electronics housing, as shown, for example, in EP-A 984 248, U.S. Pat. Nos. 4,716,770, or 6,352,000, often also serves to contain some mechanical components of the measuring transducer, such as e.g. membrane-, rod-, sleeve- or tube-shaped, deformation or vibration bodies which are deformed by mechanical effects during operation; compare, in this connection, the U.S. Pat. No. 6,352,000, mentioned above.
Furthermore, measuring systems of the type described are normally connected, via a data transfer system linked to the measuring electronics, with one another and/or with appropriate process control computers, to which they transmit the measured value signals e.g. via (4 mA to 20 mA) current loop and/or a digital data bus. Serving, in such case, as data transmission systems are fieldbus systems, especially serial fieldbus systems, such as e.g. PROFIBUS-PA, FOUNDATION FIELDBUS, along with the corresponding transmission protocols. By means of the process control computers, the transmitted measured value signals can be further processed and visualized as corresponding measurement results e.g. on monitors, and/or converted into control signals for process control elements, such as e.g. magnetic valves, electric motors, etc.
As also discussed in, among others, GB-A 21 42 725, U.S. Pat. No. 5,808,209, US-A 2005/0092101, U.S. Pat. Nos. 6,880,410, 6,644,132, 6,053,054, 6,644,132, 5,052,229 or 6,513,393, in-line measuring devices and, in this respect, also measuring systems of the type described, can, quite likely, have an accuracy of measurement dependent, more or less, on the type of flow. Of special interest in this connection is particularly the instantaneous character of the flow profile in the measuring tube. Considering that turbulent flows, thus flows with a Reynolds number greater than 2300, are largely similar to one another over a broad Reynolds number range, and, as a result, have a comparable effect on measurement accuracy, a high flow velocity of the medium to be measured is often desired in many measuring systems. To achieve a sufficiently high level of measuring accuracy in the case of vortex flow-measuring devices, one usually wants flows which have a Reynolds number much higher than 4000.
Thus, in the case of measuring systems of the type under discussion, it is quite usual, at least in the case of process lines of comparatively larger caliber and/or in applications with comparatively slower flowing media, to construct the measuring tube, if necessary, such that it has a smaller flow cross-section than a supply segment of the process line connected to the inlet side of the measuring system. As a result, the flowing medium experiences an acceleration in the flow direction, whereby, in turn, the Reynolds number is increased. Implementation of this principle has proven itself especially also in the case of measuring systems which function by means of an ultrasonic measuring device, and/or a vortex flow-measuring device, and/or which are provided for the measurement of at least partially, especially predominantly or entirely, gaseous media.
Considering that, for example, the relationship between the shedding rate of vortices on a flow-opposing, bluff body and the therewith primarily to be registered, measured variable, i.e. volume flow rate or flow velocity, which relationship forms the basis upon which vortex flow-measuring devices operate, can only first be accepted as linear to an adequate degree once a Reynolds number of 20,000 has been surpassed, it may be necessary to implement a comparatively great difference between the flow cross sections of the process line and the measuring tube. In other words, for this Reynolds number range, the Strouhal number representing the aforementioned relationship can be considered as essentially constant.
In order to create over a shortest-possible distance a transition zone which is as well-defined as possible, from the supply segment to the measuring tube with its smaller flow cross section, it is usual, as also proposed in, among others, GB-A 21 42 725, U.S. Pat. No. 5,808,209, or US-A 2005/0092101, to provide in the measuring system a flow conditioner having a lumen tapering toward the measuring tube. During operation, the medium flows through such lumen. The flow conditioner is arranged on the inlet side of the measuring tube, and mediates between the measuring tube and the supply segment of the process line. An inlet-end of the flow conditioner, facing the supply segment of the process line, has, in such case, a flow cross section which is greater than the flow cross section of the measuring tube, while an outlet-end of the flow conditioner, facing the measuring tube, accordingly has a flow cross section which is smaller than that of the inlet end.
Especially in U.S. Pat. No. 5,808,209, as well as also in US-A 2005/0092101, it is further indicated in connection with the flow conditioners presented in each case, that the transition realized between the two differently sized flow cross sections must be kept continuous and absolutely free of interruptions, such as, for example vortex-causing edges.
This can be ensured to a quite satisfactory degree by relatively complex processing of the surfaces of the flow conditioner, and of the possibly present joints in the inlet region of the measuring system. However, it has been found that, despite the use of flow conditioners of the type named above, already minor disturbances of the flow in the inlet region of the measuring system, especially also in the supply segment of the connected process line located upstream of the measuring system, or in the region of the connection flange on the inlet side, which, if needed, serves to connect the supply segment and the measuring system, are associated with a significant variation of the flow conditions inside the measuring tube lumen, and, thus, with a corresponding decline in the accuracy of measurement.
Superficially, a possibility for eliminating this problem is to perform a matching processing also of the inlet region of the measuring system, and thus of the supply segment of process line, or of the flange connection at the inlet. This processing, however, is practically impossible to carry out, at any rate not without further demands on the user of the measuring system. This especially is the case because the choice of the measuring system can result from the fact that, in an existing plant, a previously-installed and with respect to the actual flow conditions possibly over-sized measuring system is to be ad hoc replaced. In this respect, the actual installation conditions for the measuring system are to be considered not only unforeseeable, but also, as a practical matter, both not adaptable, and, as a result, also not controllable.
A further possibility for avoiding this problem is to increase the installed length of the flow conditioner in order to achieve, in the flow conditioner, a large degree of stabilizing and quieting of the flow, as early as possible, before it enters the measuring tube. However, this can mean a considerable increase in the installed length of the total measuring system. Considering the situation named above, in which an existing, conventional measuring system is to be replaced by one with a flow conditioner connected upstream, the installed length for the measuring system is more or less predetermined, and thus an increase in the installed length of the flow conditioner is possible only within this rather limited context. Given the disadvantages of conventional flow conditioners, it is no wonder that the range of application of measuring systems of the type in discussion is still seen as rather limited.